I hate to break it to you, but I don't think that'd ever be made. As far as I know, the LeapPad doesn't even have 5GHz Wi-Fi, which is how the Wii U gamepad talks to the console. And even then, it's hard on its own to figure out how to spoof the gamepad and communicate with the Wii U in such a way. Not even to mention how much extra hardware the gamepad has (gyroscope, accelerometer, magnetometer, IR transceiver, etc) that the LeapPad doesn't have.
I know this is a disappointing answer, but it's genuinely impossible.
Also, I feel like a better name would be "LeapPad U" if not just "ULeap"